Machinery's Handbook, 31st Edition
ROTARY ACTUATORS 2793 manufacturers. Manufacturers sometimes publish the C v of their actuators, or it can be calculated using the rotary swept volume. Kinetic energy is of particular concern with rotary actuators. Manufacturers normally publish the kinetic energy limits of their actua- tors. Although cushions are normally provided as part of rotary actuators, it is recom- mended to use proportional valves to decelerate rotating loads where loading is signifi - cant. Kinetic energy is calculated using the following equation: 1 2 -- I ω 2 = where w is the angular velocity (peak) in radians/sec, and I is the mass moment of inertia of the load carried on the rotary actuator shaft. KE Shock Absorbers and Hydraulic Dampers.— These devices are special types of linear actuators that are used to control applied loads. Pneumatic and hydraulic types are com mercially available. All have frequency limitations that should be examined when sizing them for an application. Pneumatic Shock Absorbers and Gas Springs: Pneumatic shock absorbers and gas springs use the compressibility of gas (usually nitrogen) to resist an applied force. Shock absorbers are used to decelerate an applied load over a distance, and are generally quite small. Gas springs are used mostly to counterbalance loads and often have very long strokes. Some gas springs have intermediate holding positions. Shock absorbers are normally used when no return assistance is required and no restric tion of velocity is required. There are some available with an orifice in the piston that provides velocity control. Shock absorbers often have a spring to provide a return function and assist in decelerating loads. They also usually have a mechanical stop, such as an adjustable nut, to prevent overcompression. Shock absorbers are sized according to impact speed, stroke, and energy absorbing ca- pacity. The energy absorbed will be the sum of the kinetic energy of the moving load and any external thrust provided by a driving mechanism. Vertical applications will also need to resist the weight of the applied load. Kinetic and thrust energies are calculated using the following equations: E total = KE + E t KE 1 2 -- mv 2 = = Kinetic energy of a linear impact, where m is mass and v is velocity. E t Fs = = Thrust energy of a linear impact, where F is applied force and s is the shock absorber stroke. KE 1 2 -- I ω 2 = = Kinetic energy of a rotary impact, where I is moment of inertia and is angular velocity. E t Ts R = --- = Thrust energy of a rotary impact, where T is applied torque and R is radius from the moment center and impact point. KE mgh = = Kinetic energy of a falling weight impact, where m is mass and h is height. E t mgs = = Thrust energy of a falling weight impact. Gas springs are used when return assistance or load support (counterbalance) is required throughout the range of motion. If the gas spring is the quick return type, the force increases as the piston moves. If an orifice is present in the piston through which the gas can escape, they can limit velocity and provide a more constant force over the full stroke.
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